Handle quotation marks in section names

This bit me today when using https://github.com/boxen/puppet-git to add an alias to a local git config file.

As https://github.com/johnsyweb/puppetlabs-inifile/commit/41fc8bfbf5d62c7daa64f330b4300375fc2609fd demonstrates, the alias should have been added to the bottom of the existing `[alias]` section, but it was actually added to the end of the file.

To fix this I have "relaxed" the `SECTION_REGEX` to be simply:

```ruby
/^\s*\[([^\]]*)\]\s*$/
```

Where all characters between the opening `[` and the closing `]` are be considered to form the section name.